To me, the tragedy of Boxer is that he gives his life to a cause that he believes in, but the cause totally just uses him and then throws him away when it has gotten what it wants.

Throughout the book, Boxer really works so hard to try to improve Animal Farm and help the regime of Animalism advance. He believes in it totally and he completely devotes his life to it. But then what happens in Chapter 9? While working to build the windmill, his lung collapses (partly because of wounds from the earlier battle). Now he is unable to work and Napoleon has him sold to the slaughterhouse.

Napoleon (who Boxer trusted and idolized) has used Boxer up and then thrown him away. This is tragic, in my mind.

Boxer was the hardest worker of all the animals who resided on Animal Farm. He was also the most dedicated to the cause of the animals, and he bought into everything that the pigs preached to him. It never occurred to him that Napoleon was taking advantage of his non-pork comrades, however, and he basically worked himself into an early grave (though, as we know, his body did not receive the dignity of interment). In the end, Boxer did not get to enjoy the retirement that was promised him; instead, he was sold to a slaughter house.

Boxer's end was very tragic. He was very dedicated and loyal to the farm, always putting in maximum effort in contributing to the farm, working his lungs out to try to improve the quality of life in the farm. He gave his own life to the Revolutionist cause, operating under his own created maxims "I will work harder" and "Napoleon is always right".

He had also battled valiantly in the Battle of the Windmill and also worked his socks out to rebuilt the windmill. But, he soon overworked himself and his lung collapsed soon after. Due to his injuries, Napoleon deemed him as a liability, as he was unable to work at all, and soon sold him to a slaughter house to be killed for glue, in exchange for bottles of whiskey.

Thus, Boxer's sudden demise was very tragic, and this shows his gullibility in trusting Napoleon.